1. Technical Field
The disclosure relates generally to metrology, and more particularly, to a method and systems for optical measurement.
2. Background Art
Optical metrology is an integral part of semiconductor research and development, and manufacturing. For example, semiconductor dimensions that may require measurements via optical metrology tools include: thickness, critical dimension (CD), sidewall angle (SWA), doping, reactive ion etching (RIE) lag, etch bias, etc., Optical metrology tools may also be used to verify lithography focus and provide exposure feedback. The benefits include non-contact measurement capability with little or no risk of damage or contamination to the sample.
One form of optical metrology is scatterometry, which generally requires a target grating along with an optical system which collects optical spectra for analysis. Targets 10, as illustrated in FIGS. 1A-B, usually consist of a 50 μm×50 μm grating 12 with a series of lines 14 and adjacent spaces (FIG. 1B simplified compared to FIG. 1A for clarity). The optical systems (not shown) used to illuminate grating 12 and collect the optical spectra generally utilize different versions of ellipsometry and/or reflectometry. As shown in FIG. 2A, ellipsometry uses an incident light beam 15 illuminating target 10 at an angle not normal to a wafer surface 16. This results in a spot which resembles an ellipse 18. As shown in FIG. 2B, reflectometry uses an incident light beam 19 illuminating target 10 at an angle normal to wafer surface 20, resulting in a substantially circular spot 22. Reflectometry often uses specific optical components, such as a polarizer (not shown), which makes the measurement sensitive to grating 10 direction. Regardless of which type of optical system is utilized the reflected beam contains the information which must be analyzed and processed to compute the desired measurement parameters using an optical model specifically created for the target being measured.
Each technique employs a variety of optics arrangements utilizing rotating and/or fixed polarizers and compensators. One problem with moving components, however, is that they oftentimes suffer from frequent mechanical and alignment problems.